Working model (scale 1:8) of a double-acting rotative beam engine, developed by Boulton and Watt in 1784, and produced until c 1800. James Watt (1736-1819) developed the first efficient steam engine. This engine incorporates Watt's separate condenser (patented in 1769) and other features including an air pump, parallel motion, and the sun-and-planet gear. This spur gear turned the up-and-down motion of the engine piston into a rotary motion to drive an axle. The gears consisted of a small 'planet' gear that revolved around the 'sun' gear wheel. By 1800, when Watt and Boulton's partnership ended, 451 engines had been built of which 268 were rotative. By the end of 19th century, the steam engine had become the main source of power for industry and transport.
© Science Museum / Science & Society Picture Library